DNA methylation and demethylation play vital roles in various aspects of mammalian development, as well as in somatic cells during differentiation and aging. Importantly, these processes are known to become highly aberrant during tumorigenesis and cancer (A. Bird, Genes Dev 16: 6-21 (2002); W. Reik, Nature 447: 425-432 (2007); K. Hochedlinger, Nature 441: 1061-1067 (2006); M. A. Surani Cell 128: 747-762 (2007); J. B. Gurdon, Annu Rev Cell Dev Biol 22: 1-22 (2006)).
In mammals, DNA methylation occurs primarily on cytosine in the context of the dinucleotide CpG. DNA methylation is dynamic during early embryogenesis and plays crucial roles in parental imprinting, X-inactivation, and silencing of endogenous retroviruses. Embryonic development is accompanied by major changes in the methylation status of individual genes, whole chromosomes and, at certain times, the entire genome (A. Bird, Genes Dev 16: 6-21 (2002); W. Reik, Nature 447: 425-432 (2007); K. Hochedlinger, Nature 441: 1061-1067 (2006); M. A. Surani Cell 128: 747-762 (2007); J. B. Gurdon, Annu Rev Cell Dev Biol 22: 1-22 (2006)). For example, there is active genome-wide demethylation of the paternal genome shortly after fertilization (W. Mayer, Nature 403: 501-502 (2000); J. Oswald, Curr Biol 10: 475-478 (2000)). DNA demethylation is also an important mechanism by which germ cells are reprogrammed: the development of primordial germ cells (PGC) during early embryogenesis involves widespread DNA demethylation mediated by an active (i.e. replication-independent) mechanism (A. Bird, Genes Dev 16: 6-21 (2002); W. Reik, Nature 447: 425-432 (2007); K. Hochedlinger, Nature 441: 1061-1067 (2006); M. A. Surani Cell 128: 747-762 (2007); P. Hajkova, Nature 452: 877-881 (2008); N. Geijsen, Nature 427: 148-154 (2004)).
De novo DNA methylation and demethylation mechanisms are also prominent in somatic cells during differentiation and aging. Expression of differentiation-specific genes in somatic cells is often accompanied by progressive DNA demethylation (W. Reik, Nature 447: 425-432 (2007); K. Hochedlinger, Nature 441: 1061-1067 (2006); M. A. Surani Cell 128: 747-762 (2007)). Tight regulation of DNA demethylation is a feature of pluripotent stem cells and progenitor cells in cellular differentiation pathways, which could contribute to the ability of these cells to self-renew, as well as give rise to daughter differentiating cells (W. Reik, Nature 447: 425-432 (2007); K. Hochedlinger, Nature 441: 1061-1067 (2006); M. A. Surani Cell 128: 747-762 (2007); J. B. Gurdon, Annu Rev Cell Dev Biol 22: 1-22 (2006); S. Simonsson Nat Cell Biol 6: 984-990 (2004); R. Blelloch, Stem Cells 24: 2007-2013 (2006)).
It is believed that two important aspects of stem cell function, pluripotency and self-renewal ability, require proper DNA demethylation, and hence, the ability to manipulate these stem cell functions could be improved by controlled expression of enzymes in the DNA demethylation pathway. The epigenetic reprogramming of somatic nuclei during somatic cell nuclei transfer (SCNT) may also require proper control of DNA demethylation pathways (W. Reik, Nature 447: 425-432 (2007); K. Hochedlinger, Nature 441: 1061-1067 (2006); M. A. Surani Cell 128: 747-762 (2007); J. B. Gurdon, Annu Rev Cell Dev Biol 22: 1-22 (2006); S. Simonsson (2004); R. Blelloch (2006)). For optimal efficiency of cloning by SCNT, regulated DNA demethylation may be required for nuclear reprogramming in the transferred somatic cell nucleus. Moreover, correct regulation of DNA demethylation could improve the efficiency with which induced pluripotent stem cells (iPS cells) are generated from adult fibroblasts or other somatic cells using pluripotency factors (K. Takahashi, Cell 126: 663-676 (2006); K. Takahashi, Cell 131: 861-872 (2007); J. Yu, Science 318: 1917-1920 (2007)).
DNA methylation processes are known to be highly aberrant in cancer. Overall, the genomes of cancer cells show a global loss of methylation, but additionally tumor suppressor genes are often silenced through increased methylation (L. T. Smith, Trends Genet. 23: 449-456 (2007); E. N. Gal-Yam, Annu Rev Med 59: 267-280 (2008); M. Esteller, Nature Rev Cancer 8: 286-298 (2007); M. Esteller, N Engl J Med 358: 1148-1159 (2008)). Thus, oncogenesis is associated with aberrant regulation of the DNA methylation/demethylation pathway. Moreover, the self-renewing population of cancer stem cells can be characterized by high levels of DNA demethylase activity. Furthermore, in cultured breast cancer cells, gene expression in response to oestrogen has been shown to be accompanied by waves of apparent DNA demethylation and remethylation not coupled to replication (R. Métivier, Nature 452: 45-50 (2008); S. Kangaspeska, Nature 452:112-115 (2008)). It is presently unknown whether this apparent demethylation is due to full conversion of 5-methylcytosine (5mC) to cytosine, or whether it reflects a partial modification of 5-methylcytosine to a base not recognized by methyl-binding proteins or antibodies to 5-methylcytosine.
DNA demethylation can proceed by two possible mechanisms—a “passive” replication-dependent demethylation, or a process of active demethylation for which the molecular basis is still unknown. The passive demethylation mechanism is fairly well understood and is typically observed during cell differentiation, where it accompanies the increased expression of lineage-specific genes (D. U. Lee, Immunity, 16: 649-660 (2002)). Ordinarily, hemimethylated CpG's are generated during cell division as a result of replication of symmetrically-methylated DNA. These hemimethylated CpGs are recognized by the DNA methyltransferase (Dnmt) 1, which then transfers a methyl group to the opposing unmethylated cytosine to restore the symmetrical pattern of DNA methylation (H. Leonhardt, Cell 71: 865-873 (1992); L. S. Chuang, Science 277: 1996-2000 (1997)). If Dnmt1 activity or localization is inhibited, remethylation of the CpG on the opposite strand does not occur and only one of the two daughter strands retains cytosine methylation.
In contrast, enzymes with the ability to demethylate DNA by an active mechanism have not been identified as molecular entities. There is evidence that active DNA demethylation occurs in certain carefully-controlled circumstances, such as shortly after fertilization, and during early development of primordial germ cells (PGC) (W. Reik, Nature 447: 425-432 (2007); K. Hochedlinger, Nature 441: 1061-1067 (2006); M. A. Surani Cell 128: 747-762 (2007); J. B. Gurdon, Annu Rev Cell Dev Biol 22: 1-22 (2006); P. Hajkova, Nature 452: 877-881 (2008); N. Geijsen, Nature 427: 148-154 (2004)). The mechanism of active demethylation is not known, though various disparate mechanisms have been postulated (reviewed in (H. Cedar, Nature 397: 568-569 (1999); S. K. Ooi, Cell 133:1145-1148 (2008)). However, no proteins with these postulated activities have been reliably identified to date.
Overall, identification of molecules that play a role in active demethylation and methods to screen for changes in the methylation status of DNA would be important for the development of novel therapeutic strategies that interfere with or induce demethylation and monitor changes in the methylation status of cellular DNA.